This invention relates to nonwoven fabrics with a high modulus of elongation. More particularly, it relates to nonwoven fabrics suitable for use in the electrical insulation field, and to a process for preparing such fabrics.
In the insulation of electrical equipment, as for example in the armature winding and slot lining of generators, maintenance of continuity of insulating value is of prime importance. The primary insulating materials, such as varnishes, epoxy resins, or mica, and the like, do not form self-sustaining manipulatable sheets of enough strength to allow their application to electrical parts. Such materials therefore are commonly mounted on a supportive base such as glass cloth, nonwoven fabrics, special papers and the like.
Supportive bases of this type must possess, as a primary requisite, a high degree of resistance to elongation or deformation under the stresses developed during the application of the insulating material and during the use of the equipment, which customarily involves elevated temperatures. If the base material stretches or deforms under low or moderate stresses, the non-elastic varnish or resin coating may develop cracks, leading to electrical leakage and loss of insulating value.
This is especially true when the primary insulating material is mica, highly desired because of its unique combination of electrical, thermal, and mechanical properties. One form of primary insulation is a mica paper, made of natural mica comminuted into finely-divided form and reconstituted into a sheet of overlapping, horizontally stratified platelets. The overlapping mica platelets form an excellent insulating medium, so long as their overlapping relationship is maintained. Such sheets, however, have little strength, and are generally mounted on a strong supportive base by means of a resin, such as an epoxy, securing the mica sheet to a glass cloth.
With the constant demand for more compact electrical components, there is a concomitant demand for insulating materials of high efficiency and decreased thickness. The fabrication of ultra-thin glass fabrics is expensive and cumbersome, so that on a practical basis, nonwoven fabrics are becoming more and more widely used in the development of thin, economical, base supports for layers of electrical insulation.
In addition to thinness, however, the nonwoven fabric must, for reasons set forth above, possess what may be called a "high modulus": that is, a high degree of resistance to elongation under stress. The modulus of elongation is derived from the slope of the elongation portion of the stress-strain curve, and for convenience herein will be referred to as the pounds of tensile stress which must be applied to a one inch wide strip of fabric to cause a specified elongation, usually between 2% and 4%.
A prior art product is shown in Davies et al, U.S. Pat. No. 3,595,731, comprising a web of polymeric material having a sheet such as nylon or polyethylene bonded to each face of the web. However, this adhesively bonded web does not have a high resistance to elongation, and the filaments (fibers) in the web are not held against displacement by the nylon or polyethylene sheets thereon. It is with improvements in the art of preparing such base fabrics that the present invention is concerned.
It is a primary object of the invention to prepare a thin, compliant nonwoven fabric with a high modulus of elongation.